The power beneath our feet

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There is one other option for the continuous production of power.
Geothermal energy has a long history, yet despite the considerable
amount of heat lying between our feet and our planet's molten
mantle, geothermal technologies provide a mere 10,000 megawatts of
power worldwide.

This sorry state of affairs may soon change, for it transpires
that we have been looking for heat in the wrong places. Previously,
geothermal power has come from volcanic regions, where aquifers
flowing through the hot rocks provide superheated water and steam.
It seems sensible to seek power in such places, but consider the
geology. Lava volcanoes only exist where the Earth's crust is being
torn apart, allowing the magma to come to the surface.

Iceland, formed from the ocean floor where Europe and North
America are drifting away from each other, is an excellent example.
There is plenty of heat in such places, but also formidable
impediments to generating power, with the biggest problem being the
aquifers. Although many flow freely when first tapped, they quickly
dwindle, leaving the power plant without a means of transferring
the rock's heat to its generators. In the 1980s operators began to
pump water back into the ground in the hope that it would be
reheated and could be reused. Quite often the water just vanished,
for in regions where the Earth's crust is being torn apart, many
vertical faults exist, and the water was diverted by these rather
than returned to the wellhead.

In Switzerland and Australia, companies are finding commercially
usable heat in the most unlikely places. When oil and gas companies
prospected in the deserts of northern South Australia, nearly four
kilometres below the surface they discovered a body of granite
heated to about 250 degrees - the hottest near-surface,
non-volcanic rock ever discovered.

The heat had been generated by the natural radioactivity of the
granite, which had been kept in place by a blanket of sediment
nearly four kilometres thick. What really excited the geologists
was that the granite was not in a region where the Earth's crust
was being torn apart, but where it was being compressed. This led
to horizontal, rather than vertical, fracturing of the rock. Even
better, the rocks are bathed in superheated water under great
pressure, and the horizontal fracturing meant that it could be
readily recycled.

This one rock body in South Australia is estimated to contain
enough heat to supply all Australia's power needs for 75 years, at
a cost equivalent to that of brown coal, without the carbon dioxide
emissions. So vast is the resource that distance to market is no
object, for power can be pumped down the power line in such volume
as to overcome any transmission losses.

With trial plants scheduled for construction this year, the
enormous potential of geothermal power is about to be tested.
Geologists around the world are scrambling to prospect for similar
deposits, as the extent of the resource is hardly known. There is
some reason to believe, however, that Australia may be specially
blessed with this type of potential power, for the continent has
been moving north at about eight centimetres a year for the past 40
million years, and when it bumped into Asia 15 million years ago,
enormous compressional forces were generated. As a result, in
Australian mines one kilometre deep, engineers must deal with
compressional forces encountered five kilometres down in places
such as South Africa.

While this appears to be an exciting breakthrough, we must
remember that so far very little electricity has been provided by
this form of geothermal heat, and even if successful, it will
probably be decades before it is contributing significantly to the
world grid.